By Tommy H. Thomason

Monday, August 31, 2015

The Gutless Cutlass?

Some airplanes just don't get any respect. The Vought F7U Cutlass suffers worse than most: "Ensign Killer", "the engines put out less heat than Westinghouse's toasters", "Gutless Cutlass", and so forth. And then there's the iconic photos and video of the VF-124 F7U-3 ramp strike:
Yes, that is the LSO sprinting across the deck to get out of the way, which he did. Contrary to what's reported in various places on the interweb, he wasn't hurt and there were no serious injuries to anyone else on deck. The pilot was killed.

In fact, the F7U-3 had about the same thrust-to-weight ratio as its contemporaries, the F9F-8 Cougar and the FJ-3 Fury, albeit in afterburner. And that's even though Westinghouse did not meet the original engine-thrust specification. The real engine problem was that "dry" engine thrust was only 2/3s of thrust in afterburner and using afterburner involved the burning of prodigious amounts of fuel, significantly limiting range and endurance.

This reliance on afterburner for "full" power would not have been quite so evident but for the fact that the Cutlass lacked a horizontal tail. It therefore required a big, thick wing for low-speed lift since no horizontal tail meant no wing flaps. This is a comparison of the F7U-1 (the F7U-3 wing was basically the same planform) and the F-86.

As a result, at low speed on a low, flat approach to an axial deck carrier, pulling the stick back to increase angle of attack actually decreased lift initially. The subsequent increase in lift was accompanied by a very large increase in drag. From the flight manual: "To change angle of attack alone results in excessive loss of altitude, and the rate of descent is so difficult to control as to preclude safe answering of altitude correction signals from the Landing Signal Officer."

So to recover from being low or settling meant adding thrust, preferably even before the pilot got low or settling began. Unfortunately, the relatively low military thrust and high drag at the angle of attack required to attain a low approach speed meant there wasn't much excess thrust unless the afterburner was used. Which was problematical with the early afterburners. From the flight manual again: "Because increases in angle of attack from flight conditions such as making a turn, stopping a rate of descent, and initiating a climb increases the drag, caution should be exercised to assure that the airplane is accelerating prior to entering the afore-mentioned maneuvers. (A waveoff) must be initiated relatively early because selection of (afterburner) is accompanied by a thrust delay lasting up to two seconds. For the first three seconds following power increase, better airplane acceleration is attained in military power; after three seconds a marked acceleration advantage is obtained in (afterburner)."

The horrific F7U-3 ramp strike was actually preceded by a routine approach and landing. The accident investigation determined that the airplane was above the maximum approach weight when it crashed, which meant it was even heavier on the first, successful, landing. However, on the second approach the pilot got low and was slow to respond to the LSO's "come on" signals.

The multiple incidences of nose gear failure (including a fatal one) were in part due to not wanting to get low and slow on approach. The appearance of being high and fast therefore tended to result in a dive for the deck at the cut, with the result being a nose-wheel first touchdown with excessive sink.
The last Cutlass deployment in the heaviest one, the missile-armed F7U-3M, was on an angled-deck carrier with a descending approach rather than a flat one. There were no incidents.

Ironically, the F8U Crusader's handling qualities on approach were at least as challenging as the F7U's and its accident rate not much better even though it only deployed on angle-deck carriers. However, supersonic performance combined with outstanding endurance meant it was revered instead of reviled and ridiculed.